Multi-engine system with on-board ammonia production

ABSTRACT

A power system is provided having a first power source including at least one engine configured to combust a first air/fuel mixture and produce a first exhaust stream. The fuel of the first air/fuel mixture may be liquefied petroleum gas. The system also has a first exhaust passageway fluidly connected to the first power source and configured to receive the first exhaust stream. In addition, the system has a second power source including at least one engine configured to combust a second fuel/air mixture and produce a second exhaust stream. Furthermore, the system has a second exhaust passageway fluidly connected to the second power source and configured to receive the second exhaust stream. The system further has a first catalyst disposed within the first exhaust passageway to convert at least a portion of the first exhaust stream to ammonia

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/806,384, filed May 31, 2007, the entire contents of whichare incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract No.DE-FC26-01CH1107 awarded by the Department of Energy. The Government mayhave certain rights in this invention.

TECHNICAL FIELD

The present disclosure is directed to a multi-engine system and moreparticularly, to a multi-engine system with at least one engine havingan on-board ammonia producing capability.

BACKGROUND

Fossil fuel powered systems for engines, factories, and power plantstypically produce emissions that contain a variety of pollutants. Thesepollutants may include, for example, particulate matter, nitrogen oxides(NOx), and sulfur compounds. Due to heightened environmental concerns,exhaust emission standards have become increasingly stringent. Theamount of pollutants in the exhaust stream may be regulated depending onthe type, size, and/or class of engine.

One method used to reduce emissions is selective catalytic reduction(SCR). SCR provides a method for removing NOx emissions from internalcombustion engine systems. During SCR, a catalyst facilitates a reactionbetween a reductant (e.g., ammonia) and NOx to produce water vapor andnitrogen gas, thereby removing NOx from the exhaust gas. Ammonia that isused for the SCR system may be stored for injection when needed.However, because of the high reactivity of ammonia, storage can beproblematic. In addition, machines utilizing SCR systems sometimesoperate in remote locations where it may be difficult to replenish theammonia. On-board ammonia production may provide a safer and morepractical alternative to ammonia storage.

U.S. Pat. No. 5,964,088 (the '088 patent) issued to Kinugasa et al. onOct. 12, 1999, discloses two embodiments of a system utilizing on-boardammonia production. One embodiment disclosed in the '088 patent includesa multi-cylinder engine that combusts a lean air/fuel mixture. A firstcylinder of the engine is fluidly connected to an exhaust passagewaythat has an ammonia synthesizing catalyst, while the other cylinders arefluidly connected to an SCR catalytic device. A separate auxiliaryengine combusts a rich air/fuel mixture and is fluidly connected to theexhaust passageway with the ammonia synthesizing catalyst. Rich exhaustgas from the auxiliary engine is mixed with lean exhaust gas from thefirst cylinder, and NOx contained in the mixture reacts with the ammoniasynthesizing catalyst to generate ammonia. The ammonia is then directedto the SCR catalytic device where it reacts with the lean exhaust of theremaining cylinders to reduce NOx.

In a second embodiment, all of the engine cylinders combust a leanair/fuel mixture and are fluidly connected to an SCR catalytic device.The separate auxiliary engine is replaced with a burner that burns arich air/fuel mixture and is fluidly connected to an ammoniasynthesizing catalyst. NOx in the rich exhaust gas produced by theburner reacts with the ammonia synthesizing catalyst, and the resultingammonia is directed to the SCR catalytic device. There, the ammonia ismixed with the lean exhaust produced by the engine cylinders and reactswith the SCR catalytic device to remove NOx from the engine emissions..

Although the system in the '088 patent may reduce NOx emissions, theutilization of lean exhaust gas or a burner to generate ammonia maylimit the NOx reducing capability of the system. In particular, withrespect to the first embodiment, the lean exhaust gas contains a largeamount of oxygen which adversely affects the production of ammonia.Thus, the combination of the rich exhaust gas with the lean exhaust gasbefore the exhaust gas is converted to ammonia limits the amount ofammonia that can be produced. With respect to the second embodiment, theburner combusts the rich air/fuel mixture at a temperature that isunfavorable for NOx production, thereby limiting ammonia generation. NOxreduction in both embodiments is limited because only a limited amountof ammonia is available to react with the SCR catalytic device.

In addition, the engine system disclosed in the '088 patent consumes alarger amount of fuel to produce a particular mechanical or electricaloutput than a conventional power system without on-board ammoniaproduction. This is because additional fuel is needed to power theauxiliary engine and the burner, which do not contribute to theproduction of the mechanical or electrical output. Therefore, energyfrom the additional fuel is used solely to produce ammonia and is notused to accomplish the task being performed by the main engine. By usingthe additional fuel, operational costs may increase and the systemefficiency may decrease.

The disclosed system is directed to overcoming one or more of theproblems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure is directed toward a power systemthat includes a first power source including at least one engineconfigured to combust a first air/fuel mixture and produce a firstexhaust stream. The fuel of the first air/fuel mixture may be liquefiedpetroleum gas. The system also includes a first exhaust passagewayfluidly connected to the first power source and configured to receivethe first exhaust stream. In addition, the system includes a secondpower source having at least one engine configured to combust a secondair/fuel mixture and produce a second exhaust stream. Furthermore, thesystem includes a second exhaust passageway fluidly connected to thesecond power source and configured to receive the second exhaust stream.The system further includes a first catalyst disposed within the firstexhaust passageway to convert at least a portion of the first exhauststream to ammonia.

The present disclosure is also directed to a locomotive that includes afirst power source configured to power the locomotive. The first powersource may include at least one engine configured to combust a firstair/fuel mixture. The locomotive may also include a second power sourceconfigured to power the locomotive. The second power source may includeat least one engine configured to combust a second air/fuel mixture. Thelocomotive may further include at least one generator drivingly coupledto at least one of the first power source or the second power source.The at least one generator may be configured to generate electricalenergy to power the locomotive. The locomotive may also include a firstexhaust passageway fluidly connected to the first power source andconfigured to receive a first exhaust stream and a second exhaustpassageway fluidly connected to the second power source and configuredto receive a second exhaust stream. The locomotive may include a firstcatalyst disposed within the first exhaust passageway to convert atleast a portion of the first exhaust stream to ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed machine;

FIG. 2 is a diagrammatic illustration of an exemplary disclosed powersystem for use with the machine of FIG. 1;

FIG. 3 is a diagrammatic illustration of another embodiment of theexemplary disclosed machine;

FIG. 4 is a schematic illustration of another exemplary embodiment ofthe disclosed power system; and

FIG. 5 is a flow chart depicting an exemplary method for operating thepower system.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary machine 10 having multiple systems andcomponents that cooperate to accomplish a task. The tasks performed bymachine 10 may be associated with a particular industry such as mining,construction, farming, transportation, power generation, or any otherindustry known in the art. For example, machine 10 may embody a mobilemachine such as, a bus, a haul truck, a locomotive, a marine vessel, orany other type of machine known in the art. As shown in FIG. 1, machine10 may include one or more traction devices 12 operatively connected toand driven by a power train 14.

Traction devices 12 may embody wheels located on each side of machine 10(only one side shown). Alternatively, traction devices 12 may includetracks, belts or other known traction devices. It is contemplated thatany combination of the wheels on machine 10 may be driven and/orsteered.

Power train 14 may be an integral package configured to generate andtransmit power to traction devices 12. In particular, as shown in FIG.2, power train 14 may include a power system 16 operable to generate apower output and a transmission unit 18 connected to receive the poweroutput and transmit the power output to traction devices 12.

Power system 16 may include a first power source 20 configured tocombust a rich air/fuel mixture and a second power source 22 configuredto combust a lean air/fuel mixture. First power source 20 may include atleast one rich engine 24, while second power source 22 may include atleast one lean engine 26. Rich engine 24 and lean engine 26 may benatural gas powered engines. Rich engine 24 and lean engine 26 mayalternatively be any other type of internal combustion engine such as,for example, a gasoline, a diesel, or a gaseous fuel-powered engine.First power source 20 may be operationally connected to second powersource 22 by, for example, a countershaft 28, a belt (not shown), anelectrical circuit (not shown), or in any other suitable manner suchthat first power source 20 and second power source 22 cooperativelycontribute to produce a mechanical or electrical output. It iscontemplated that in configurations utilizing multiple rich engines 24and/or multiple lean engines 26, each rich engine 24 may beoperationally connected to other rich engines 24 and lean engines 26 maybe operationally connected to other lean engines 26 by, for example,countershaft 28, a belt (not shown), an electrical circuit (not shown),or in any other suitable manner such that all rich engines 24 and leanengines 26 cooperatively contribute to produce a mechanical orelectrical output. It is further contemplated that although first powersource 20 and second power source 22 are disclosed as being situated inseries, first and second power sources 20 and 22 may be disposed in aparallel configuration. It is yet further contemplated that rich engine24 may embody an auxiliary power unit.

Power system 16 may have multiple subsystems that cooperate to produce amechanical or electrical power output. Among such subsystems includedwithin power system 16 may be an exhaust system 30 and a control system32.

Exhaust system 30 may remove or reduce the amount of pollutants in theexhaust produced by power system 16 and release the treated exhaust intothe atmosphere. Exhaust system 30 may include an exhaust passageway 34fluidly connected to an exhaust manifold 36 of first power source 20, anammonia-producing catalyst 38 disposed within exhaust passageway 34, anexhaust passageway 40 fluidly connected to an exhaust manifold 42 ofsecond power source 22, a merged exhaust passageway 44 fluidly connectedto exhaust passageways 34 and 40, and a selective catalytic reduction(SCR) catalyst 46 disposed within merged exhaust passageway 44 (exhaustpassageways 34 and 40 may merge at SCR catalyst 46 or upstream of SCRcatalyst 46). It is contemplated that exhaust system 30 may furtherinclude additional after-treatment devices, such as, for example, one ormore oxidation catalysts 48, an ammonia oxidation catalyst 50, one ormore particulate filters 52, and/or any other after-treatment deviceknown in the art that is capable of removing or reducing unwantedemissions from the exhaust, if desired.

Ammonia-producing catalyst 38 may generate ammonia by facilitating areaction between NOx and other combustion byproducts in the exhaust-gasstream of first power source 20. These other combustion byproducts mayinclude, for example, hydrogen gas (H₂), propene (C₃H₆), or carbonmonoxide (CO). In addition, ammonia-producing catalyst 38 may include avariety of materials, such as, for example, platinum, palladium,rhodium, iridium, copper, chrome, vanadium, titanium, iron, cesium, orany other material capable of generating ammonia. Combinations of thesematerials may be used, and the catalyst material may be chosen based onthe type of fuel used, the air/fuel ratio desired, or for conformitywith environmental standards.

The efficiency of the ammonia-producing reaction may be improved underrich conditions. Therefore, the air/fuel mixture combusted within firstpower source 20 may be made rich to generate a rich exhaust favorablefor increased ammonia production. Alternatively, a fuel-supply device(not shown) may be fluidly connected to exhaust passageway 34 upstreamof ammonia-producing catalyst 38 and configured to supply fuel intoexhaust passageway 34. The injection of fuel into the exhaust of firstpower source 20 may produce favorable conditions for generating ammonia.

SCR catalyst 46 may facilitate a reaction between the ammonia generatedby ammonia-producing catalyst 38 and NOx to at least partially removeNOx from the exhaust stream in merged exhaust passageway 44. Forexample, SCR catalyst 46 may facilitate a reaction between the ammoniaand NOx to produce nitrogen gas and water, among other reactionproducts.

Oxidation catalyst 48 may be situated within exhaust passageway 40 andmay regulate the levels of different NOx components in the exhaust ofsecond power source 22 to increase the performance of SCR catalyst 46.It is contemplated that a plurality of oxidation catalysts 48 mayalternatively be situated within each exhaust passageway 40 of secondpower source 22, if desired. NOx may include several oxides of nitrogenincluding nitrogen oxide (NO) and nitrogen dioxide (NO₂). However, SCRcatalyst 46 may function most effectively with a NO:NO₂ ratio of 1:1.Therefore, oxidation catalyst 48 may be used to oxidize NO into NO₂ toregulate the ratio of NO to NO₂ in the exhaust stream of second powersource 22 and increase the performance of SCR catalyst 46.

Ammonia oxidation catalyst 50 may be situated within merged exhaustpassage 44 downstream of SCR catalyst 46 and may oxidize or burn anyexcess ammonia that may pass through SCR catalyst 46. During the exhausttreatment process, ammonia may be generated and supplied to SCR catalyst46 at a rate that may exceed the NOx reducing capacity of SCR catalyst46. The excess ammonia, known as ammonia slip, may be expelled from SCRcatalyst 46 and may contribute to undesired emissions released into theatmosphere. In addition, the excess ammonia may corrode the surfaces ofexhaust treatment equipment located downstream of SCR catalyst 46, whichcan lead to maintenance issues. Ammonia oxidation catalyst 50 mayprevent such issues by converting the excess ammonia to nitrogen gas(N₂).

Particulate filter 52 may be situated within exhaust passageway 34,exhaust passageway 40, and/or merged exhaust passageway 44 to removeparticulate matter from the exhaust flow. It is contemplated thatparticulate filter 52 may include a catalyst for reducing an ignitiontemperature of the particulate matter trapped by particulate filter 52,a means for regenerating the particulate matter trapped by particulatefilter 52, or both a catalyst and a means for regenerating. The meansfor regenerating may include, among other things, a fuel-powered burner,an electrically-resistive heater, an engine control strategy, or anyother means for regenerating known in the art.

Control system 32 may regulate the air/fuel ratio of an air/fuel mixturecombusted by first and second power sources 20 and 22 based on sensedNOx and ammonia levels in exhaust system 30. By regulating the air/fuelratio, first and second power sources 20 and 22 may generate an optimalamount of NOx and ammonia for exhaust treatment. Control system 32 mayinclude a NOx sensor 54 situated within exhaust passageway 34 upstreamof ammonia-producing catalyst 38 and/or an ammonia sensor 56 situatedwithin exhaust passageway 34 downstream of ammonia-producing catalyst38. Control system 32 may also include a NOx sensor 58 situated withinexhaust passageway 40 and a controller 60. It should be understood thatalthough FIG. 2 discloses that control system 32 includes three sensors,any number of sensors and any combination of sensors may be used.Furthermore, the sensors may be located anywhere within power system 16that may adequately sense the amount of NOx and ammonia in exhaustsystem 30. For example, it is contemplated that control system 32 mayinclude additional NOx sensors situated within merged exhaust passageway44 either upstream or downstream of SCR catalyst 46. It is alsocontemplated that control system 32 may include virtual sensors.

NOx sensor 54 may sense the amount of NOx generated by first powersource 20 and may be mounted on exhaust passageway 34 upstream ofammonia-producing catalyst 38. In addition, NOx sensor 54 may beconfigured to detect the level of NOx in the exhaust flow passingthrough exhaust passageway 34. At least a portion of NOx sensor 54 mayextend through the wall of exhaust passageway 34 into the exhaust flow.In order to withstand the high temperatures in exhaust passageway 34,NOx sensor 54 may be constructed, for example, out of ceramic type metaloxides or any other suitable material. NOx sensor 54 may sample theexhaust for NOx, and convert that sensed value into a signal indicativeof the NOx level therein.

Ammonia sensor 56 may sense the amount of ammonia generated byammonia-producing catalyst 38 and may be mounted on exhaust passageway34 downstream of ammonia-producing catalyst 38. In addition, ammoniasensor 56 may be configured to detect the level of ammonia in theexhaust flow passing through exhaust passageway 34. At least a portionof ammonia sensor 56 may extend through the wall of exhaust passageway34 into the exhaust flow. In order to withstand the high temperatures inexhaust passageway 34, ammonia sensor 56 may be constructed, forexample, out of ceramic type metal oxides or any other suitablematerial. Ammonia sensor 56 may sample the exhaust for ammonia, andconvert that sensed value into a signal indicative of the ammonia leveltherein.

NOx sensor 58 may sense the amount of NOx generated by second powersource 22 and may be mounted on exhaust passageway 40. In addition, NOxsensor 58 may be configured to detect the level of NOx in the exhaustflow passing through exhaust passageway 40. At least a portion of NOxsensor 58 may extend through the wall of exhaust passageway 40 into theexhaust flow. In order to withstand the high temperatures in exhaustpassageway 40, NOx sensor 58 may be constructed, for example, out ofceramic type metal oxides or any other suitable material. NOx sensor 58may sample the exhaust for NOx, and convert that sensed value into asignal indicative of the NOx level therein.

Controller 60 may include one or more microprocessors, a memory, a datastorage device, a communication hub, and/or other components known inthe art and may be associated only with first and second power sources20 and 22. However, it is contemplated that controller 60 may beintegrated within a general control system capable of controllingadditional functions of power system 16, e.g. and/or additionalsubsystems operatively associated with power system 16, e.g., selectivecontrol of transmission unit 18.

Controller 60 may receive signals from NOx sensors 54, 58 and ammoniasensor 56 and analyze the data to determine the amount of NOx andammonia in the exhaust gas. Upon receiving input signals from NOxsensors 54, 58 and ammonia sensor 56, controller 60 may perform aplurality of operations, e.g., algorithms, equations, subroutines,reference look-up maps or tables to determine whether the NOx andammonia levels are optimal and establish an output to influence theair/fuel ratio of the air/fuel mixture combusted by engines 24 and 26.Alternatively, it is contemplated that controller 60 may receive signalsfrom various sensors (not shown) located throughout power system 16instead of NOx sensors 54, 58 and ammonia sensor 56. Such sensors maysense parameters that may be used to calculate the amount of NOx andammonia in exhaust system 30.

Transmission unit 18 may include numerous components that interact totransmit power from power system 16 to traction device 12. Inparticular, transmission unit 18 may be a multi-speed bidirectionalmechanical transmission having a neutral gear ratio, a plurality offorward gear ratios, a reverse gear ratio, and one or more clutches (notshown). The clutches may be selectively actuated to engage predeterminedcombinations of gears (not shown) to produce a desired output gearratio. It is contemplated that transmission unit 18 may be anautomatic-type transmission, with shifting based on a power sourcespeed, a maximum selected gear ratio, and a shift map, or a manual-typetransmission, with shifting between each gear directly initiated by anoperator. The output of transmission unit 18 may be connected to andconfigured to rotatably drive traction device 12 via output shaft 62,thereby propelling machine 10.

It is contemplated that transmission unit 18 may alternately embody ahydraulic transmission having one or more pumps and hydraulic motors, ahydro-mechanical transmission having both hydraulic and mechanicalcomponents, an electric transmission having a generator and one or moreelectric motors, an electromechanical transmission having bothelectrical and mechanical components, or any other suitabletransmission. It is also contemplated that transmission unit 18 mayalternately embody a continuously variable transmission such as, forexample, an electric transmission having a generator and an electricmotor, a hydraulic transmission having a pump and a fluid motor, or anyother continuously variable transmission known in the art.

FIG. 3 illustrates another exemplary embodiment of machine 10. In theembodiment of FIG. 3, machine 10 may embody a land based machine (e.g.,a locomotive, a truck, a bus, etc.), a marine vessel, or a stationarypower generation application. In this embodiment, rich engine 24 may bepowered by liquefied petroleum gas (“LPG”), and lean engine 26 may bepowered by diesel fuel. Alternatively, rich engine 24 and lean engine 26may be powered by gasoline, diesel, ethanol, JP-5, JP-8, gaseous fuel(e.g., propane, butane, dimethyl ether, natural gas, or any combinationthereof), or any other type of fuel known in the art. It is contemplatedthat rich engine 24 may receive fuel (e.g., LPG) from another vehicle66. For example, if machine 10 embodies a locomotive, tank 64 may belocated on a tender car 66. Tank 64 may alternatively be located onmachine 10.

The power output of rich engine 24 and lean engine 26 may be connectedto one or more generators 68 to produce electrical energy. Eachgenerator 68 may be a device configured to produce a power output inresponse to a rotational input provided by an engine (e.g., rich engine24 or lean engine 26). It is contemplated that generator 68 may embody,for example, a permanent magnet-type generator, an asynchronousgenerator, or any other type of generator configured to produce eitheralternating current or direct current electrical energy. Generator 68may include a rotor (not shown) rotatably connected to an engine (e.g.,rich engine 24 or lean engine 26) by any means known in the art, suchas, for example, a direct crankshaft connection, a driveshaft, a geartrain, a hydraulic circuit, or in any other appropriate manner.

The electrical energy produced by generators 68 may be used, forexample, to power a motor 70 (or multiple motors 70) for propulsion ofmachine 10 and any other vehicles associated with machine 10 (e.g.tender car 66). Each motor 70 may be an electric motor configured toreceive power from generator 68 and create rotation of traction devices12. It is contemplated that motors 70 may be direct current motors,alternating current motors, or any other appropriate type of motorsknown in the art. In one embodiment, an output of motors 70 may beconnected to traction devices 12 via a gear mechanism (not shown). Otherelectrical components (not shown) may be associated with generator 68and motors 70, such as rectifiers, inverters, and other electricalcomponents known in the art.

It should be understood that the configuration of exhaust system 30illustrated in FIG. 3 may be similar to the embodiment disclosed in FIG.2.

FIG. 4 illustrates another exemplary embodiment of power system 16 usedin applications such as, for example, powering marine vessels, poweringland-based vehicles, and various industrial applications. In theexemplary embodiment of FIG. 4, rich engines 24 of first power source 20may operate independently of each other to produce separate mechanicalor electrical outputs. In addition, lean engines 26 of second powersource 22 may operate independently of each other to produce separatemechanical or electrical outputs. Furthermore, first power source 20 andsecond power source 22 may operate independently of each other toproduce separate mechanical or electrical outputs. It should beunderstood that the configuration of exhaust system 30 illustrated inFIG. 4 may be similar to the embodiment disclosed in FIG. 2.

FIG. 5, which is discussed in the following section, illustrates theoperation of first and second power sources 20 and 22 utilizingembodiments of the disclosed system. Specifically, FIG. 5 illustrates anexemplary method for regulating the air/fuel ratio of the air/fuelmixture combusted by engines 24 and 26 for optimal exhaust emissionlevels.

INDUSTRIAL APPLICABILITY

The disclosed multi-engine system may reliably and efficiently remove orreduce NOx emissions from exhaust that is released into the atmosphere.In particular, the disclosed multi-engine system may eliminate the needfor peripheral equipment such as burners or storage tanks to supplyammonia necessary for NOx reduction to the exhaust treatment system. Bydesignating one engine or set of engines to facilitate the generation ofammonia, the multi-engine system itself may supply the ammonia requiredto remove or reduce NOx emissions from the exhaust released into theatmosphere. The operation of first and second power sources 20 and 22will now be explained.

FIG. 5 illustrates a flow diagram depicting an exemplary method forgenerating an optimal level of NOx and ammonia in the exhaust to meetemission standards. The method may begin when the air/fuel mixture to becombusted by second power source 22 is set to a desired air/fuel ratio(step 100). The desired ratio may be any ratio capable of producing adesired result related to the operation of first and second powersources 20 and 22. Such desired results may include, for example, fuelefficiency or maximum mechanical or electrical power generation. Inaddition, it should be understood that the air/fuel ratio of theair/fuel mixture entering second power source 22 may be leaner thanstoichiometric. Furthermore, the air/fuel ratio may be regulated by anymethod known in the art such as, for example, adjusting the setting of athrottling valve (not shown).

Once the air/fuel mixture is set to the desired ratio, controller 60 mayreceive signals indicative of the amount of NOx and ammonia in exhaustsystem 30 from NOx sensors 54 and 58 and ammonia sensor 56 (step 102).Controller 60 may compare the sensed amount of NOx to tables, graphs,and/or equations stored in its memory to determine whether the sensedamount of NOx is below a predetermined threshold (step 104). Such athreshold may be related to government regulated emissions limits or anyother threshold related to the amount of emissions released into theatmosphere. If controller 60 determines that the sensed amount of NOx isbelow the predetermined threshold (step 104: YES), step 102 may berepeated (i.e. controller 60 may receive new signals from NOx sensors54, 58 and ammonia sensor 56 indicative of new NOx and ammonia levels).However, if controller 60 determines that the amount of NOx is above thepredetermined threshold (step 104: No), controller 60 may determinewhether the amount of ammonia in exhaust system 30 is above apredetermined threshold for exhaust treatment (step 106).

The predetermined threshold may be dependant upon the amount of NOx inexhaust system 30. For example, the desired amount ammonia may increasewhen the amount of NOx increases and decrease when the amount of NOxdecreases. In addition, controller 60 may determine the desired amountof ammonia for a particular amount of NOx by referencing look-up mapsand/or tables and/or performing algorithms, equations, or subroutines.If controller 60 determines that the amount of ammonia in exhaust system30 is below the predetermined threshold (step 106: No), controller 60may adjust the amount of NOx being produced by first power source 20 toreduce the generation of ammonia (step 108). For example, controller 60may decrease the amount of NOx by decreasing the power output of firstpower source 20. The power output may be decreased by reducing theamount of air and fuel entering first power source 20. It should beunderstood that, regardless of the power output, the air/fuel mixturebeing combusted by first power source 20 may be maintained at a constantair/fuel ratio that is richer than stoichiometric. It is contemplatedthat other techniques may be employed to reduce the amount of NOxproduced by first power source 20. Such techniques may include, forexample, adjusting the timing of combustion. Once the amount of NOxbeing produced has been adjusted, step 102 may be repeated (i.e.controller 60 may receive new signals from NOx sensors 54, 58 andammonia sensor 56 indicative of new NOx and ammonia levels).

If controller 60 determines the amount of ammonia in exhaust system 30is below predetermined threshold (step 106: Yes), then controller 60 maydetermine whether ammonia-producing catalyst 38 is operating at itsmaximum capacity (step 110). Controller 60 may make this determinationby referencing look-up maps and/or tables and/or performing algorithms,equations, or subroutines. If controller 60 determines thatammonia-producing catalyst 38 is operating below its maximum capacity(step 110: No), controller 60 may adjust the amount of NOx beingproduced by first power source 20 to increase the generation of ammonia(step 112). For example, controller 60 may increase the amount of NOx byboosting the power output of first power source 20. The power output maybe boosted by increasing the amount of air and fuel entering first powersource 20. It should be understood that, regardless of the power output,the air/fuel mixture being combusted by first power source 20 may bemaintained at a constant air/fuel ratio that is richer thanstoichiometric. It is contemplated that other techniques may be employedto increase the amount of NOx produced by first power source 20. Suchtechniques may include, for example, adjusting the timing of combustion.Once the amount of NOx being produced has been adjusted, step 102 may berepeated (i.e. controller 60 may receive new signals from NOx sensors54, 58 and ammonia sensor 56 indicative of new NOx and ammonia levels).

If controller 60 determines that ammonia-producing catalyst 38 isoperating at its maximum capacity (step 110: Yes), controller 60 mayreduce the amount of NOx being produced by second power source 22 (step114). For example, controller 60 may decrease the amount of NOx bydecreasing the power output of second power source 22. The power outputmay be decreased by reducing the amount of air and fuel entering secondpower source 22. It should be understood that, regardless of the poweroutput, the air/fuel mixture being combusted by second power source 22may be maintained at a constant air/fuel ratio that is leaner thanstoichiometric. It is contemplated that other techniques may be employedto reduce the amount of NOx produced by second power source 22. Suchtechniques may include, for example, adjusting the timing of combustion.Once the amount of NOx being produced has been adjusted, step 102 may berepeated (i.e. controller 60 may receive new signals from NOx sensors54, 58 and ammonia sensor 56 indicative of new NOx and ammonia levels).

The disclosed system may generate as much ammonia as required to reduceor remove NOx emissions from exhaust released into the atmosphere.Because any oxygen present in the ammonia-producing catalyst may hinderproduction of ammonia and limit the amount produced, it may be desiredto minimize amount of oxygen in the ammonia-producing catalyst. Byisolating the rich (low-oxygen) exhaust of the engine or set of enginesdesignated for facilitating the generation of ammonia from the lean(high-oxygen) exhaust generated by the other engine or set of enginesthe amount of oxygen present in the ammonia-producing catalyst may beminimized. In addition, the engine or set of engines designated forfacilitating ammonia production may be configured to combust a richair/fuel mixture at temperatures that are conducive for NOx production,which is necessary for ammonia generation.

In addition, the disclosed system may consume a substantially similaramount of fuel to produce a particular mechanical or electrical outputas a conventional multi-engine system without on-board ammoniaproduction. This is because exhaust used to generate the ammonia may beproduced by engines that contribute to the production of the mechanicaland electrical output of the system. Therefore, an additional separatesupply of fuel is not necessary for ammonia production, thereby reducingcosts and increasing efficiency of the system.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the disclosed system withoutdeparting from the scope of the disclosure. Other embodiments will beapparent to those skilled in the art from consideration of thespecification disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope beingindicated by the following claims and their equivalents.

1. A power system, comprising: a first power source including at leastone engine configured to combust a first air/fuel mixture and produce afirst exhaust stream, the fuel of the first air/fuel mixture beingliquefied petroleum gas; a first exhaust passageway fluidly connected tothe first power source and configured to receive the first exhauststream; a second power source including at least one engine configuredto combust a second air/fuel mixture and produce a second exhauststream; a second exhaust passageway fluidly connected to the secondpower source and configured to receive the second exhaust stream; and afirst catalyst disposed within the first exhaust passageway to convertat least a portion of the first exhaust stream to ammonia.
 2. The powersystem of claim 1, wherein the fuel of the second air/fuel mixture isdiesel fuel.
 3. The power system of claim 2, wherein the first andsecond exhaust passageways are fluidly connected downstream from thefirst catalyst to form a merged exhaust passageway configured to receivea combined exhaust stream.
 4. The power system of claim 3, furtherincluding a second catalyst disposed within the merged exhaustpassageway.
 5. The power system of claim 4, wherein the second catalystis configured to facilitate a reaction between ammonia and NOx in thecombined exhaust stream to at least partially remove NOx from thecombined exhaust stream.
 6. The power system of claim 5, furtherincluding at least one sensor configured to sense a parameter indicativeof an amount of NOx in the first and/or second exhaust passageways andat least one sensor configured to sense a parameter indicative of anamount of ammonia in the first exhaust passageway.
 7. The power systemof claim 6, further including a controller configured to adjust theamount of NOx produced by the first and/or second power sources inresponse to the sensed amount of NOx and/or ammonia.
 8. The power systemof claim 2, wherein the first air/fuel mixture is richer thanstoichiometric condition.
 9. The power system of claim 8, wherein thesecond air/fuel mixture is leaner than stoichiometric condition.
 10. Thepower system of claim 2, wherein the first power source and the secondpower source are configured to power at least one of a locomotive or amarine vessel.
 11. A locomotive, comprising: a first power sourceconfigured to power the locomotive, the first power source including atleast one engine configured to combust a first air/fuel mixture; asecond power source configured to power the locomotive, the second powersource including at least one engine configured to combust a secondair/fuel mixture; at least one generator drivingly coupled to at leastone of the first power source or the second power source, wherein the atleast one generator is configured to generate electrical energy to powerthe locomotive; a first exhaust passageway fluidly connected to thefirst power source and configured to receive a first exhaust stream; asecond exhaust passageway fluidly connected to the second power sourceand configured to receive a second exhaust stream; and a first catalystdisposed within the first exhaust passageway to convert at least aportion of the first exhaust stream to ammonia.
 12. The locomotive ofclaim 11, wherein the fuel of the first air/fuel mixture is diesel fuel,and the fuel of the second air/fuel mixture is liquefied petroleum gas.13. The locomotive of claim 12, wherein the second power source receivesthe liquefied petroleum gas from a tender car.
 14. The locomotive ofclaim 11, wherein the first and second exhaust passageways are fluidlyconnected downstream from the first catalyst to form a merged exhaustpassageway configured to receive a combined exhaust stream.
 15. Thelocomotive of claim 14 further including a second catalyst disposedwithin the merged exhaust passageway.
 16. The locomotive of claim 15,wherein the second catalyst is configured to facilitate a reactionbetween ammonia and NOx in the combined exhaust stream to at leastpartially remove NOx from the combined exhaust stream.
 17. Thelocomotive of claim 16, further including at least one sensor configuredto sense a parameter indicative of an amount of NOx in the first and/orsecond exhaust passageways and at least one sensor configured to sense aparameter indicative of an amount of ammonia in the first exhaustpassageway.
 18. The locomotive of claim 11, wherein the first air/fuelmixture is richer than stoichiometric condition and the second air/fuelmixture is leaner than stoichiometric condition.
 19. A machine,comprising: a first power source configured to power the machine, thefirst power source including at least one engine configured to combust afirst air/fuel mixture; a second power source configured to power themachine, the second power source including at least one engineconfigured to combust a second air/fuel mixture, wherein the first powersource and the second power source generate a mechanical power output; afirst exhaust passageway fluidly connected to the first power source andconfigured to receive the a first exhaust stream; a second exhaustpassageway fluidly connected to the second power source and configuredto receive a second exhaust stream; and a first catalyst disposed withinthe first exhaust passageway to convert at least a portion of the firstexhaust stream to ammonia.
 20. The machine of claim 19, wherein the fuelof the first air/fuel mixture and the fuel of the second air/fuelmixture is natural gas.